Purple loosestrife (Lythrum salicaria)

Wetland perennial, 3’-7’ tall, with up to 50 stems topped with purple flower spikes. One main leader stem, but many side branches often make the plant look bushy. Clipped plants grow back, and cut stems readily re-root in soil to produce new plants. Many areas of the state use safe biocontrol beetles that feed on the loosestrife to keep it in check and allow other plants to grow.

Species Assessment Groups (SAG) were assembled to recommend a legal classification for each species considered for NR 40. The recommendation for purple loosestrife was based upon this literature review developed by the department.

Identification

Leaves: simple, lance-shaped and do not have petioles. Usually opposite and rotated 90 degrees from those below, but are sometimes whorled.

Flowers: closely attached to the stem with 5-6 pink-rose colored petals. Blooms from the bottom of the flower spike to the top from early July to September. Plants can bloom the first year after seeds germinate.

Fruits & seeds: capsules burst open when mature in late June to September. A single stem can produce 100,000-300,000 seeds per year. Mature plants with many stems can produce 2 million seeds. Seeds are viable for at least 7 years.

Roots: large woody taproot and many side roots. Plants intertwine to form dense clumps.

Control

Mechanical: Young, small plants can be dug or pulled. Larger plants can be dug if all root fragments are removed. Burn, landfill, or bury all plant parts deep in the ground. Mowing is not recommended as plant parts may re-sprout and seeds may be dispersed.

Chemical: Imazapyr or glyphosate work well against purple loosestrife. If near water a permit may be required and aquatic-use formulas of these herbicides should be used.

Biological: Galerucella beetles have been successful in many parts of the state in controlling purple loosestrife populations. Want to get involved with biocontrol? Find out more on our purple loosestrife biocontrol page.